Forgive my ignorance - can you buy paracetamol capsules that could be opened up and doctored with the Zopiclone, I haven't seen these? I think he would have been most likely to tamper with her vitamin/herbal capsules, safe in the knowledge that she took those daily (presumably she only took paracetamol sporadically).
There are quite a lot of herbal capsule remedies on the market, and some such as ginseng are surprisingly large - emptied out he could no doubt have easily put 2-4 crushed Zopiclone into one capsule. For Helen to have been doing things like collapsing on a beach and going home leaving her beloved Boris behind, I feel he must have been giving her massive doses. So he must have had more tablets than the 28 stated from the GP prescription. No doubt his computer and phone history could have revealed he was placing big orders online.